The sig banned message is passing a loud message to who ever sees it. You commit plagiarism, you get punished so I don't support the idea of removing it.
I wasn't even aware of this until I read OP's post. I have signatures blocked, so I don't see any of them and I figured that when members got a signature ban, their sig field would just be blank. Interesting that Theymos decided to put a message there instead--and I agree with you, it does send a loud & clear message about plagiarism. Obviously either don't read the rules or don't think they're going to get caught copy/pasting, so maybe this is exactly what we need.
Their sig field is blank only when they will post and on all of their previous post. This message is only displayed when you click to view profile. The message will be loud and clear when you make this ban message as their sig. message on all the post they posted till now.